Two crew members have died after a US Navy helicopter crashed in the Atlantic off the coast of Virginia.

They are among four who were rescued from the cold waters - but lost their lives in hospital several hours later.

Rescuers called off the search for a fifth crew member on Thursday after searching an area of 500 square miles by air and sea for more than 30 hours.

Captain John Little, Coast Guard section commander, said the frigid temperatures in the Atlantic contributed to the decision to end the active rescue effort.

Ocean temperatures in the area were 6C (42F) on Wednesday, when the MH-53E Sea Dragon was forced to make an emergency landing about 20 miles east of Virginia Beach.

According to the Personal Flotation Device Manufacturers Association, an adult can survive one to three hours in 4C to 10C (40 to 50F) water and would become exhausted or unconscious between 30 and 60 minutes.

The sailors who died have been identified as Lt Wesley Van Dorn, 29, and 25-year-old Petty Officer 3rd Class Brian Collins.

The remaining rescued personnel are in a stable condition, the Navy said.

"Today has definitely been a tough day on all of us," said Captain Todd Flannery, the commander of Helicopter Sea Combat Wing Atlantic.

Image:The scene of Tuesday's deadly helicopter crash in Norfolk

"Our heartfelt prayers go out to the families and loved ones of those killed and injured in today's crash."

The Navy said the helicopter had been conducting a routine training mission when it went down.

The cause of the accident is under investigation.

The crash is the second deadly US military helicopter accident in as many days.